Seismic surveying is used for identifying subterranean elements, such as hydrocarbon reservoirs, fresh water aquifers, gas injection reservoirs, and so forth. In performing seismic surveying, seismic sources and seismic sensors can be placed at various locations on an earth surface (e.g., a land surface or a sea floor), or even in a wellbore, with the seismic sources activated to generate seismic waves. Examples of seismic sources include explosives, air guns, acoustic vibrators, or other sources that generate seismic waves.
Some of the seismic waves generated by a seismic source travel into a subterranean structure, with a portion of the seismic waves reflected back to the surface (earth surface, sea floor, or wellbore surface) for receipt by seismic sensors (e.g., geophones, hydrophones, etc.). These seismic sensors produce signals that represent detected seismic waves. Signals from the seismic sensors are processed to yield information about the content and characteristics of the subterranean structure.
A portion of a seismic wave generated by a seismic source travels along the surface. If the surface is assumed to be horizontal, then this seismic wave portion travels horizontally along the surface. Such a seismic wave portion is referred to as a surface seismic wave, which is also referred to as ground roll noise.
In one example, as depicted in FIG. 1 (which shows a top view of an arrangement of a seismic source and seismic sensors), multiple lines of seismic sensors (lines 102 and 104) are provided, where each seismic sensor is represented as “G.” The seismic source (vibrator) is represented as “V,” and is referenced as 100. Each line 102, 104 of seismic sensors is a linear array of geophones in the example. As further depicted in FIG. 1, surface seismic waves 106, 108, and 110 (representing ground roll in different horizontal directions) are depicted.
FIG. 2 illustrates the ground roll surface seismic wave 108 propagating along the direction of a linear array (102 or 104) of geophones (G1-G6), where the source V and geophones G1-G6 are placed on a surface 111 (e.g., land surface or sea floor). Also, FIG. 2 illustrates propagation of seismic waves (116) into a subterranean structure 112 underneath the surface 111. The subterranean structure 112 includes a target reflector 114 (which can be a hydrocarbon reservoir, water aquifer, gas injection zone, etc.). The target reflector 114 reflects seismic waves towards the geophones G1-G6.
In the horizontal direction along the line of the geophones G1-G6, the ground roll seismic wave 108 arrives at the geophones G1-G6 at different times. In other words, the ground roll seismic wave 108 arrives at geophone G1 first, and at geophone G6 last. However, the reflected seismic wave from the target reflector 114 arrives at the geophones G1-G6 almost at the same time (with some small difference).
Summation of the traces represented by signals detected by the individual geophones G1-G6 allows for attenuation of the ground roll surface wave 108 that propagates along the linear direction of the line of geophones G1-G6. However, the linear arrangement of the geophones of FIGS. 1 and 2 does not provide for ground roll attenuation for ground roll seismic waves traveling in the crossline direction (directions 106 and 110) in FIG. 1, where a crossline direction is the direction of wave propagation that is perpendicular to the line of seismic sensors. Ground roll attenuation is not possible or effective with the linear arrangement of geophones as depicted in FIGS. 1 and 2 because the crossline ground roll seismic waves arrive at the geophones at substantially the same time. Moreover, it is not cost-efficient to position additional geophones along the crossline direction, as planting geophones on an earth surface or in a wellbore is a time-consuming and labor-intensive operation.
Another issue associated with a ground roll surface seismic wave is that it exhibits dispersive characteristics (different velocities at different seismic signal frequencies), which can make attenuation difficult. Also, the ground roll surface seismic wave can continuously change its form due to dispersion as the seismic wave propagates.
Moreover, even in the linear direction of a line of seismic sensors, a relatively large number of seismic sensors usually have to be provided in the linear array due to the relatively high velocity of the ground roll surface seismic wave. A long linear array of seismic sensors can degrade imaging resolution, because reflected seismic waves may come in at an oblique angle.